Process and device for recovering argon by low-temperature separation of air

ABSTRACT

The process and the device are used for recovering argon by low-temperature separation of air. Process air ( 1 ) is compressed ( 3 ) and is introduced into a distilling-column system for nitrogen-oxygen separation ( 13, 14 ). An argon-containing stream ( 72 ) is removed in the distilling-column system for nitrogen-oxygen separation. The argon-containing stream ( 72 ) is sent to a crude argon column ( 25 ). The crude argon column ( 25 ) has a top condenser ( 24 ), in which a top gas from the crude argon column ( 25 ) is at least partially condensed. At least one part of the condensate that is recovered in this case is released as reflux liquid to the crude argon column ( 25 ). A crude argon stream ( 76, 176, 276   a,    276   b ) is removed from the crude argon column ( 25 ) or the top condenser ( 24 ). The crude argon stream ( 76, 176, 276   a,    276   b ) is sent to a pure argon column ( 20 ). A pure argon product stream ( 81 ) is removed from the pure argon column ( 20 ). The top condenser ( 24 ) of the crude argon column ( 25 ) is designed as a reflux condenser, and top gas from the crude argon column is introduced into the reflux passages of the reflux condenser.

The invention relates to a process and apparatus for recovering Argon from air.

Processes and devices for low-temperature separation of air are known from, for example, Hausen/Linde, Tieftemperaturtechnik [Low-Temperature Technology], 2^(nd) Edition 1985, Chapter 4 (pages 281 to 337). The distillation-column system for nitrogen-oxygen separation of the invention can be designed as a one-column system for nitrogen-oxygen separation, as a two-column system (for example as a standard Linde double-column system), or else as a three-column or multiple-column system. In addition to the columns for nitrogen-oxygen separation and for argon recovery, additional steps can be provided in the process to recover other air components, in particular additional noble gases.

A top condenser is generally the only source of reflux liquid for the crude argon column. This top condenser can be formed from one, two or multiple plate heat exchanger blocks, which are connected in parallel on the evaporation side and the liquefaction side. Coolant flows into one end in the evaporation passages and out the other end from the evaporation passages. A countercurrent within the evaporation passages does not take place. Rather, coolant that remains liquid and evaporated coolant are conveyed within the evaporation passages in co-current flow. If the top condenser is designed as a bath evaporator, the coolant flows into the evaporation passages from the bottom to the top.

Processes for argon recovery of the above mentioned type are known from, for example, DE 2325422 A, EP 171711 A2, EP 377117 B2 (=U.S. Pat. No. 5,019,145), DE 4030749 A1, EP 628777 B1 (=U.S. Pat. No. 5,426,946), EP 669508 A1 (=U.S. Pat. No. 5,592,833), EP 669509 B1 (=U.S. Pat. No. 5,590,544), EP 942246 A2, EP 1103772 A1, DE 196049490 (=U.S. Pat. No. 5,669,237), FIG. 8, EP 1243882 A1 (=US 2002178747 A1) and EP 1243881 A1 (=US 2002189281 A1); the two—the crude argon column of the conventional condenser is used as a top condenser—are conveyed in co-current flow into the gas to be condensed and the condensed liquid in the liquefaction passages.

An object of the invention is to indicate a process of the above-mentioned type and a corresponding device that can be operated especially advantageously in economic terms by having an elevated product yield, a higher product purity, lower operating costs, and/or lower investment costs.

This object is achieved in that the top condenser of the crude argon column is designed as a reflux condenser, and top gas from the crude argon column is introduced into the reflux passages of the reflux condenser. Upon further study of the specification and appended claims, other objects and advantages of the invention will become apparent.

Thus, the present invention comprises a process for recovering argon by low-temperature separation of air, in which

-   -   Process air (1) is compressed (3) and is introduced into a         distillation-column system for nitrogen-oxygen separation (13,         14),     -   An argon-containing stream (72) is removed from such         distillation-column system for nitrogen-oxygen separation,     -   The argon-containing stream (72) is sent to a crude argon column         (25),     -   The crude argon column (25) has a top condenser (24), formed by         at least one plate heat exchanger block, which has liquefaction         passages and evaporation passages,     -   A top gas from the crude argon column (25) is introduced into         the liquefaction passages of the top condenser and is at least         partially condensed there,     -   At least one part of the condensate that is recovered in this         case is released as reflux liquid to the crude argon column         (25),     -   The top condenser represents the only source for reflux of the         crude argon column,     -   A liquid coolant is sent to the evaporation passages at a first         end and is partially evaporated there,     -   A mixture consisting essentially of evaporated coolant and         coolant that remains in liquid form is drawn off from a second         end of the evaporation passages,     -   A crude argon stream (76, 176, 276 a, 276 b) is removed from the         crude argon column (25) or the top condenser (24),     -   The crude argon stream (76, 176, 276 a, 276 b) is sent to a pure         argon column (20), and     -   A pure argon product stream (81) is removed form the pure argon         column (20),

the improvement wherein the top condenser (24) of the crude argon column (25) is as a reflux condenser having reflux passages, and top gas from the crude argon column is introduced into the reflux passages of the reflux condenser.

In the context of the claimed invention, the “crude argon column” is used for argon-oxygen separation and can be formed by a one-piece column or else by a two- or multiple-piece column, as is described in EP 628777 B1. Conversely, the “pure argon column” is used for argon-nitrogen separation. The “crude argon stream” has a higher argon concentration than the “argon-containing stream.” The “pure argon product stream” has a higher argon concentration than the crude argon stream and is preferably drawn off from the lower area of the pure argon column, for example from the bottom thereof.

“Reflux condenser” (also referred to as dephlegmator) is defined here as a heat exchanger that has reflux passages. These reflux passages are flushed from below with vapor (here: top gas from the crude argon column). The latter at least partially condenses when rising in the reflux passages. The reflux passages in this case are designed such that the condensed liquid is not entrained but rather flows downward. Rectification is accomplished by the countercurrent of vapor and liquid in the reflux passages. The condensate, which exits at the lower end, is enriched in less-volatile components, and the vapor that exits above is enriched in more-volatile components.

Various designs of reflux condensers are known. The heat exchanger block (or else a multiple of heat exchanger blocks) can be arranged inside a pressure vessel, as is shown in, for example, EP 1189000 A2, or the heat exchanger block is closed on all sides by headers, see, for example, U.S. Pat. No. 6,128,920. As an alternative to this, the reflux condenser can be installed at the top of a separation column (here: the crude argon column), whereby the reflux passages at the lower end thereof are in communication with the upper area of the separation column; see German Patent Application 102006037058 and corresponding applications. The heat exchanger block(s) of the reflux condenser is (are) preferably designed as a plate heat exchanger, in particular as a soldered aluminum plate heat exchanger.

Spatial terms such as “above,” “below,” “lateral,” etc., always relate here to the orientation of the reflux condenser in the operation in accordance with the specifications.

A reflux condenser makes possible not only a heat exchange, but also a mass transfer between the gas that rises in the reflux passages and the liquid that flows downward there, similar to the serrated packages of a packed column. This separation effectiveness can be indicated as an HETP value (Height Equivalent to One Theoretical Plate=height of one theoretical plate). The HETP value of the condenser is in the range of 300 to 600 mm. Thus, for example, a reflux condenser with a height of 1.5 m produces, for example, up to five theoretical plates. At the top of the crude argon column, however, this effect is not exerted on argon-oxygen separation, i.e., the use of the reflux condenser does not reduce the number of mass transfer elements (actual plates, orderly packing, or random tower packing) in the crude argon column.

Heretofore, a reflux condenser was previously only used as a top condenser in argon recovery if no crude argon, but rather pure argon, is recovered in the column, which is connected directly to the distillation-column system for nitrogen-oxygen separation (see U.S. Pat. No. 5,133,790). In that case, in a single argon column, both the argon-oxygen separation and the argon-nitrogen separation are performed, and a separate pure argon column for argon-nitrogen separation is expressly not provided. The upper area of the argon column from U.S. Pat. No. 5,133,790 is not used for argon-oxygen separation (as in the crude argon column of the present invention), but rather for argon-nitrogen separation, which is performed virtually exclusively in the pure argon column in the process of the initially mentioned type.

To date, no reason was thus seen, in processes that have a separate pure argon column for argon-nitrogen separation, to move a portion of the argon-nitrogen separation in the crude argon column or the top condenser thereof, since as a result, no plates can be eliminated in the columns.

Within the scope of the invention, however, it has been discovered that the use of such a reflux condenser at the top of the crude argon column has another advantage. Such condensers are regularly designed as condenser-evaporators. Thus, on the evaporation side, a coolant is evaporated from the top gas that condenses on the liquefaction side (reflux passages). The heat exchanger block is usually arranged in a bath. Because of the hydrostatic pressure, the temperature in the evaporation passages rises from the top to the bottom.

By the separating action of the reflux condenser at the top of the crude argon column, the gas that flows upward into the reflux passages is increasingly nitrogen-rich and is the coldest at the top of the condenser because of the increased proportion of nitrogen (see FIG. 4). Thus, the temperature variation in the reflux passages is matched to that of the evaporation passages. In this way, the reflux condenser has a natural tendency to develop an almost uniform driving temperature gradient over the entire block height. In the conventional crude argon condenser, however, the driving temperature gradient in the lower condenser range is always much smaller than in the upper range. This reduces the contribution of the heating surface located in the lower condenser section in the overall heat exchange. In the process according to the invention, however, the temperature difference between evaporation and liquefaction passages is almost constant. Thus, the exchange cooling losses can be reduced, or the exchange surface area is correspondingly decreased and thus the investment costs are reduced.

The product purity and/or product yield are thus increased. Consequently, the number of theoretical plates in the crude argon column can be reduced; thus, the investment costs of the unit are decreased.

It is especially advantageous if, in the invention, the liquid coolant is sent to the evaporation passages on the lower end thereof, and the mixture that consists of evaporated coolant and the coolant that remains liquid is drawn off from the lower end of the evaporation passages. For example, the top condenser is designed as a bath evaporator, in which the evaporation passages are open at the top and bottom and the coolant is conveyed by means of the thermosiphon effect from the bottom to the top through the evaporation passages.

In one embodiment of the invention, the top condenser is formed by exactly one plate heat exchanger block.

In the process according to the invention, it is especially advantageous if the crude argon stream is drawn off from the upper area of the reflux passages. The gaseous fraction that remains after flushing has an especially high argon concentration, and its oxygen content is especially low. The crude argon stream thus also contains a relatively large amount of nitrogen; the latter can be separated in the pure argon column, however, without great expense.

In another embodiment of the invention, no residual gas stream is drawn off from the upper area of the crude argon column and from the reflux passages. Preferably, no additional stream at all is drawn off from the upper area including the reflux passages in addition to the crude argon stream. For example, in the addition to the crude argon stream, only one additional stream, which is returned to the distilling-column system for nitrogen-oxygen separation (for example in the low-pressure column of a two-column system, from which the argon-containing stream is also drawn off), is drawn off from the crude argon column.

It is also advantageous if the crude argon stream is removed in gaseous form from the crude argon column or the top condenser and is condensed at least partially, for example completely, upstream from its introduction into the pure argon column in an additional condenser. For this purpose, the crude argon stream can be introduced at least partially, for example completely, in liquid form into the pure argon column.

This additional condenser as well as the following measures can also be used in processes in which the top condenser is not designed as a reflux condenser.

Preferably, the top condenser and the additional condenser are designed as condenser-evaporators, whereby both evaporation passages are fed with the same coolant. The coolant is partially evaporated in the evaporation passages, whereby liquid is entrained by the thermosiphon effect and is returned into the liquid bath. As a coolant, for example, oxygen-enriched liquid from the distilling-column system is used for nitrogen-oxygen separation, for example from the bottom of the high-pressure column of a two-column system.

It is also advantageous if the top condenser and the additional condenser are designed as a liquid bath evaporator and are arranged in the same liquid bath. Since the additional condenser routinely has a lower height than the top condenser, the additional condenser can still be operated at a temperature on the lower end that is less than the temperature on the lower end of the top condenser.

If the crude argon stream in the liquid state is released at the top of the pure argon column, the latter can be used as a reflux liquid, and a top condenser of the pure argon column can be eliminated. This is known in the art from U.S. Pat. No. 5,970,743 and U.S. Pat. No. 6,574,988 B1. In combination with the reflux condenser, however, an elevated argon yield is produced.

It is also advantageous if a residual gas stream is drawn off from the top of the pure argon column or from the upper area of the reflux passages, and the charging air, in particular before its compression, is mixed in. This recycling of the residual gas from the top of the pure argon column or the crude argon column can also be applied advantageously in the argon recovery process without a reflux condenser at the top of the crude argon column. Unlike the discarding of the residual gas, the argon that is contained in the latter is recycled in the process. The argon yield increases accordingly. In principle, a separate recompressor can be used in this case, but feeding into the depressurized process air upstream from the air compressor is more advantageous, in particular more advantageous than the direct recycling of the residual gas in the distilling-column system for nitrogen-oxygen separation, as it is proposed in U.S. Pat. No. 5,133,790.

In addition, the invention relates to a device according to Claims 12 to 15.

BRIEF DESCRIPTION OF DRAWINGS

The invention as well as additional details of the invention are explained in more detail below based on the embodiments that are depicted diagrammatically in the drawings. In this connection:

FIG. 1 shows a first embodiment of the process according to the invention without an additional condenser,

FIG. 2 shows a second embodiment with an additional condenser,

FIG. 3 shows the crude argon column and the pure argon column of a third embodiment,

FIG. 4 shows the temperature and concentration gradient in a crude argon column-top condenser, which is designed according to the invention as a reflux condenser,

FIG. 5 shows a diagrammatic longitudinal section through a reflux passage of a special design of a reflux condenser, and

FIG. 6 shows another special design of the reflux condenser with a longitudinal section through a reflux passage.

Components or process steps that correspond to one another bear the same reference numbers in the drawings.

In the following description of the depicted embodiments, it is to be understood that such description is not intended to be limitative of the appended claims.

In the process of FIG. 1, atmospheric air 1 is suctioned off via a filter 2 from an air compressor 3 and compressed there to an absolute pressure of 5.0 to 7.0 bar, preferably about 5.5 bar, and is then cooled with cooling water 5, 6 in a direct contact condenser 4 in direct heat exchange, which, on the one hand (5), originates from an evaporative condenser 7, and, on the other hand (6), is fed from another source. The compressed and cooled air 8 is purified in a purification device 9, which has a pair of vessels that are filled with adsorption material, preferably molecular sieve. The purified air 10 is cooled to approximately dew point in a main heat exchanger system 11 a, 11 b, 11 c. The cold air 12 is introduced into the high-pressure column 13 of a distilling-column system for nitrogen-oxygen separation, which in addition has a low-pressure column 14. High-pressure column 13 and low-pressure column 14 are designed as a standard Linde double column and are connected via a main condenser 15 to exchange heat. The operating pressures—in each case at the top—are 4.5 to 6.5 bar, preferably about 5.0 bar in the high-pressure column and 1.2 to 1.7 bar, preferably about 1.3 bar, in the low-pressure column.

Liquid crude oxygen 16 is drawn off from the bottom of the high-pressure column 13, subcooled in a subcooling countercurrent device 17 and further cooled in a part 19 in a bottom evaporator 21 of the pure argon column 20. Another part 22 can be routed around to the bottom evaporator 21. Then, a part 23 flows into the evaporation chamber of a top condenser 24 of a crude argon column 25, and another part flows into the evaporation chamber of a top condenser 27 of the pure argon column 20. The crude oxygen 28, 29 that is evaporated in the top condensers 24, 27 is fed via line 30 of the low-pressure column 14 to a first intermediate point. The portion 31 from the top condenser 24 of the crude argon column 25 that remains liquid is also conveyed to the first intermediate point of the low-pressure column 14. The portion 32 that remains liquid from the top condenser 27 of the pure argon column 20 is released to a second intermediate point of the low-pressure column 14, which is above the first intermediate point.

Gaseous nitrogen 33 from the top of the high-pressure column 13 is directed to a first part 34 in the cold end of the main heat exchanger 11 a, heated there to approximately ambient temperature, and then divided into a pressure product stream 36 (GAN I) and a circulatory stream 37. The circulatory stream 37 is compressed in a circulatory compressor 38 with secondary condenser 39 to a pressure of 25 to 60 bar, preferably about 35 bar, and cooled in the main heat exchanger 11 a. A part 40 of the high-pressure nitrogen is removed from the main heat exchanger at an intermediate temperature and is actively depressurized in an expansion turbine 41 to about high-pressure column pressure. The depressurized circulatory stream 42 is mixed in again with the cold 34 pressure product stream. Optionally present liquid is separated in advance (43) and released via line 44 to the top of the low-pressure column 14. Another part 61 of the high-pressure nitrogen is conveyed up to the cold end of the main heat exchanger 11 a and then released to the high-pressure column 13.

The residual gaseous top nitrogen 45 of the high-pressure column 13 is at least partially condensed in the main condenser 15. The liquid nitrogen 46 that is produced in this case is released as reflux to a part 47 of the high-pressure column 13. Another part 48, 49 is directed to the top of the low-pressure column 14 after subcooling in the subcooling countercurrent device 17. There, a portion 50 can be drawn off as liquid nitrogen product (LIN).

Directly above the bottom of the low-pressure column 14, gaseous oxygen 51 is removed, heated in the main heat exchanger 11 a and drawn off via line 52 as a depressurized gaseous product (GOX III). A liquid oxygen stream 53 from the bottom of the low-pressure column 14 is subcooled in the subcooling countercurrent device 17 and sent via line 54 to a liquid tank (LOX). At least a portion of the liquid oxygen is removed again via line 55 to the tank, brought to the required product pressure in a pump 56, for example 6 to 60 bar, preferably about 31 bar, and evaporated from high-pressure nitrogen in the main heat exchanger 11 a (or pseudo-evaporated at supercritical pressure) and heated to ambient temperature, and finally drawn off via line 57 as a gaseous high-pressure product (GOX I). A portion 58 of the high-pressure liquid is depressurized via a butterfly valve 59 to an intermediate pressure of, for example, 6 to 25 bar, preferably about 15 bar, and evaporated below this lower pressure and drawn off via line 60 as a gaseous medium-pressure product (GOX II).

Gaseous nitrogen 62, 63, 64 from the top of the low-pressure column 14 and gaseous impure nitrogen 65, 66, 67 from an intermediate point of the low-pressure column 14 are in each case subcooled in a subcooling countercurrent device 17, heated in the main heat exchanger block 11 c or 11 b, and introduced via line 68—optionally after heating 69—as a regeneration gas for the purification device 9, fed via line 70 to the evaporative condenser 70 and/or drained off directly into the atmosphere via line 71.

At a third intermediate point, which is arranged below the first intermediate point, an argon-containing stream 72 is removed from the low-pressure column 14 and is fed directly over the bottom to the crude argon column 25. The crude argon column 25 is made in one piece in this example. Bottom liquid 73 of the crude argon column is returned via pump 74 and line 75 into the low-pressure column.

The top condenser 24 of the crude argon column 25 is designed according to the invention as a reflux condenser. Gas from the top of the crude argon column 25 flows down into the reflux passages and is partially condensed there. The condensate that is produced in this case flows downward in countercurrent to the rising gas in the reflux passages and is used in the crude argon column 25 as a liquid reflux. On the evaporation side, the top condenser 24 is designed as a bath condenser. The coolant, which is formed here by liquid crude oxygen 23, flows down over one or more lateral openings into the evaporation passages and is partially evaporated there. Liquid is entrained by the thermosiphon effect, it exits together with the evaporated portion on the upper end of the evaporation passages and is returned into the liquid bath. The top condenser is thus designed as a bath evaporator on the evaporation side.

From the upper end of the reflux passages, a crude argon stream 76 is removed in gaseous form via a lateral header, and is sent to the pure argon column 20 at an intermediate point. The top condenser of the pure argon column 20 is conventionally designed on the liquefaction side in the example, i.e., the top gas 77 of the pure argon column 20 flows from the top to the bottom through the liquefaction passages. (As an alternative, the top condenser 27 of the pure argon column 20 and/or the main condenser 15 could also be designed as reflux condensers.) A residual gas stream 78 is drawn off from the top condenser 27 and blown off into the atmosphere in the example. As an alternative, it can be recycled via a separate fan into the distillation-column system for nitrogen-oxygen separation or in front of the air compressor 3.

The bottom liquid 79 of the pure argon column 20 is evaporated to form a part 80 in the bottom evaporator 21, and the vapor 81 that is produced in this case is used as a rising gas in the pure argon column 20. The residue is removed as a liquid pure argon product stream 82.

The embodiment of FIG. 2 deviates from FIG. 1 primarily in the design of the pure argon column 20. Here, the pure argon column does not have a top condenser. Here, the crude argon stream 176 is formed by a part of the crude argon column 25 from the reflux passages of the top condenser 24 and released at the top of the pure argon column 20. The top gas 177 of the pure argon column 20 is recycled to the top of the crude argon column 25. The residual gas stream 178 is formed by the portion of top gas, not condensed in the top condenser 24, from the crude argon column and the pure argon column. On the upper end of the reflux passages, it is removed via a lateral header and can be treated like the residual gas stream 78 of FIG. 1.

FIG. 3 shows only the crude argon column 25 and the pure argon column 20. Otherwise, the process is identical to that of FIGS. 1 and 2. Like FIG. 2, here, a first crude argon stream 276 a is introduced in liquid form into the pure argon column 20. Deviating from FIG. 2, this introduction is not carried out at the top, but rather, as in FIG. 1, at an intermediate point of the pure argon column 20. At this point, a part 277 of the gas rising in the pure argon column is also removed and recycled to the top of the crude argon column 25.

The vapor 276 b from the top of the reflux passages of the top condenser 24 forms a second crude argon stream. The latter is at least partially condensed in an additional condenser 227, which is designed as a condenser-evaporator. The condensate 282 is released as a reflux to the top of the pure argon column. The evaporation side of the additional condenser 227 is designed like that of the top condenser 24 as a liquid bath evaporator, whereby the two are preferably arranged in the same liquid bath, which is fed by liquid crude oxygen 23.

In FIG. 4, on the one hand, the temperature gradient is plotted over the height of the condenser block (left axis). Between the liquid that condenses in the reflux passages—whose temperature is approximately the same as the temperature in the evaporation passages (upper “condensation” curve)—and the evaporating gas that rises in countercurrent thereto (lower “evaporation” curve), a temperature difference (MTD), which is almost constant over the height of the reflux condenser, prevails.

On the other hand, the nitrogen content of the gas that rises in the block is also shown. In the example, the reflux condenser has used a separating action of five theoretical plates. In the top condenser of a crude argon column, one theoretical plate produces a nitrogen concentration by approximately the factor of 3 (K-value of nitrogen in argon).

All described condensers are preferably designed as soldered aluminum plate heat exchangers, whose channels contain corrugated sheets, so-called fins. Within the reflux passages, basically the same types of fins can be used. However, it may be advantageous to use different types of fins in reflux condensers. One embodiment is shown in FIG. 5. The reflux passages that are shown here are divided into four sections A to D, in which different fin types are used. In the intake area of the reflux condenser found below, the gas leakage and thus the tendency toward flooding are at a maximum. In the upward direction, the gas leakage is always less. Therefore, preferably a fin with a small specific pressure loss and a relatively poor heat transfer is selected in the lower area A. In the upward direction, fins with greater pressure loss and better heat transfer in each case are used in the areas B, C and D. For example, the wavelength of the fins (fin density) increases toward the top.

FIG. 6 shows another method for configuring the operation of the reflux passages of the reflux condenser 24, such that the gas leakage that decreases toward the top is basically compensated for. In this case, a portion of the gas that is to be condensed is released upward to the reflux passages via line 383. This reduces the gas leakage in the lower zone. Since the amount of gas that flows toward the top is only a partial amount of the amount of gas to be condensed, the required tubing occupies little space and structural volume is reduced.

In another embodiment, the reflux condenser is configured on the evaporation side as a falling-film evaporator, i.e., the coolant that is to be evaporated is released at the top and flows downward in a film flow through the evaporation passages. Also, this produces especially advantageous plots of the evaporation and liquefaction temperatures over the height of the reflux condenser.

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

In the foregoing and in the examples, all temperatures are set forth uncorrected in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated.

The entire disclosures of all applications, patents and publications, cited herein and of corresponding German application No. 102007035619.8, filed Jul. 30, 2007 are incorporated by reference herein.

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. 

1. In a process for recovering argon by low-temperature separation of air, in which Process air (1) is compressed (3) and is introduced into a distillation-column system for nitrogen-oxygen separation (13, 14), An argon-containing stream (72) is removed from said distillation-column system for nitrogen-oxygen separation, The argon-containing stream (72) is sent to a crude argon column (25), The crude argon column (25) has a top condenser (24), formed by at least one plate heat exchanger block, which has liquefaction passages and evaporation passages, A top gas from the crude argon column (25) is introduced into the liquefaction passages of the top condenser and is at least partially condensed there, At least one part of the condensate that is recovered in this case is released as reflux liquid to the crude argon column (25), The top condenser represents the only source for reflux of the crude argon column, A liquid coolant is sent to the evaporation passages at a first end and is partially evaporated there, A mixture consists essentially of evaporated coolant and coolant that remains in liquid form is drawn off from a second end of the evaporation passages, A crude argon stream (76, 176, 276 a, 276 b) is removed from the crude argon column (25) or the top condenser (24), The crude argon stream (76, 176, 276 a, 276 b) is sent to a pure argon column (20), and A pure argon product stream (81) is removed form the pure argon column (20), the improvement wherein the top condenser (24) of the crude argon column (25) is a reflux condenser having reflux passages, and top gas from the crude argon column is introduced into the reflux passages of the reflux condenser.
 2. A process according to claim 1, wherein the liquid coolant is returned to the evaporation passages at their lower end, and the mixture that consists essentially of evaporated coolant and coolant that remains in liquid form is drawn off from the lower end of the evaporation passages.
 3. A process according to claim 1, wherein the top condenser is formed by a single plate heat exchanger block.
 4. A process according to claim 1, wherein the crude argon stream (76, 276 b) is drawn off from the upper area of the reflux passages.
 5. A process according to claim 1, wherein no residual gas stream is drawn off from the upper area of the crude argon column (25) and from the reflux passages.
 6. A process according to claim 1, wherein the crude argon stream (276 b) is removed from the crude argon column or the top condenser (24) in gas form and is at least partially condensed upstream from its introduction (282) into the pure argon column (20) in an additional condenser (227).
 7. A process according to claim 6, wherein the top condenser (24) and the additional condenser (227) are condenser-evaporators, whereby the two evaporation passages are fed with the same coolant (23).
 8. A process according to claim 7, wherein the top condenser (24) and the additional condenser (227) are liquid bath evaporators and are arranged in the same liquid bath.
 9. A process according to claim 1, wherein the crude argon stream (176, 282) is released in the liquid state to the top of the pure argon column (20).
 10. A process according to claim 1, wherein a residual gas stream (78, 178, 278) is drawn off from the top of the pure argon column (20) or from the upper area of the reflux passages, and mixed with the process air, before compression (3).
 11. A process according to claim 1, wherein the reflux condenser comprises at least one heat exchanger block, in the form of a plate heat exchanger.
 12. Apparatus for recovering argon by low-temperature separation of air comprising An air compressor (3) for compressing process air (1), Conduit for introducing the compressed process air (8, 10, 12) into a distillation-column system for nitrogen-oxygen separation (13, 14), Conduit for removal of an argon-containing stream (72) from the distillation-column system for nitrogen-oxygen separation, Conduit for introducing the argon-containing stream (72) into a crude argon column (25), Said crude argon column (25) being mounted with a top condenser (24), is formed by at least one plate heat exchanger block, having liquefaction passages and evaporation passages, Means for introducing a top gas from the crude argon column (25) into the liquefaction passages of the top condenser, Means for releasing at least a portion of the condensate that is recovered in the top condenser (24) as a reflux liquid to the crude argon column (25), Whereby the top condenser represents the only source for reflux of the crude argon column, Means for introducing a liquid coolant into the evaporation passages at a first end of the top condenser, Means for removing a mixture that consists essentially of evaporated coolant and coolant that remains in liquid form from the evaporation passages of a second end of the top condenser, Conduit for removing a crude argon stream (76, 176, 276 a, 276 b) from the crude argon column (25) or the top condenser (24), Means Conduit for introducing the crude argon stream (76, 176, 276 a, 276 b) into a pure argon column (20), Means for removing a pure argon product stream (81) from the pure argon column (20), wherein the top condenser (24) of the crude argon column (25) is a reflux condenser having means for introducing top gas of the crude argon column into the reflux passages of the reflux condenser.
 13. Apparatus according to claim 12, wherein the means for introducing a liquid coolant into the evaporation passages at the lower end of the top condenser and the means for removing a mixture that consists of evaporated coolant and coolant that remains in liquid form from the evaporation passages are arranged at the lower end of the top condenser.
 14. Apparatus according to claim 12, wherein the top condenser is comprises a single plate heat exchanger block.
 15. A process according to claim 13, wherein the crude argon stream (76, 276 b) is drawn off from the upper area of the reflux passages. 